Method and device for determining the tension or the tensile stress in a metal strip

ABSTRACT

Method and device for determining the tension or the tensile stress in a metal strip between two roll stands, between a roll stand and a coiler, in a breakdown stand or in front of or behind a deflection roll, the excursion of the metal strip being measured, and the natural frequency of the metal strip being determined on the basis of the excursion of the metal strip.

[0001] The invention relates to a device and a method for determining the tensile stress or the strip tension in a metal strip between two bearing points, for example between two roll stands, between a roll stand and a coiler, between a roll stand and a deflection roll, between two deflection rolls or between the rolls in a stretching and tension-leveling unit, with sensors for measuring the oscillation of the metal strip.

[0002] The use of a deflection roll which is arranged between a roll stand and a coiler, and over which a metal strip is passed, is known. Arranged under the bearings of the deflection roll are force sensors, which measure the bearing forces. The forces acting in the direction of the strip are determined from the bearing force components and the deflection angle. In the case of small strip tensions and small deflection angles, the system is severely affected by errors, since the measuring errors of the small forces and small deflection angles that are measured lead to considerable errors in the conversion to the strip tension.

[0003] In a further known configuration, the torque which acts in the drive train between the coiler drum and the motor shaft is measured. It is disadvantageous in this case that the great measuring range of the strip tension is further extended by the possible ratio between maximum and minimum coil radius. In this case, too, the errors are very great in the case of a small strip tension and a small coil diameter, so that the accuracy requirements can only be satisfied with high outlay.

[0004] Like any measuring method, both methods have a series of sources of error, which ultimately lead to measuring errors of the order of magnitude of 1 to 2% in relation to the measuring range end value. In particular in the case of rolling thin end thicknesses on reversing roll stands (for example foil roll stands), the ratio of maximum to minimum strip tension between the roll stand and the coiler at a stand may be of the order of magnitude of 50. Taking into account the requirement that, even with minimal strip tension, a measuring accuracy of at least 10% of the strip tension at the time must be achieved, the limits of feasible industrial measuring technology are reached, since, if these still just tolerable errors are related to the maximum strip tension or the still higher measuring range end value, the relative measuring error under the conditions must be only of the order of magnitude of 1%.

[0005] The aim of the invention is to obviate these disadvantages and, in particular, realize a measuring method which ensures a significant improvement in the measuring accuracy, in particular in the range of small strip tensions.

[0006] The object is achieved by a device as claimed in claim 1 and a method as claimed in claim 14. Developments are specified in the respective subclaims.

[0007] In this case, for determining the tensile stress or the tension in a metal strip between two bearing points, for example between two roll stands, between a roll stand and a coiler, between a roll stand and a deflection roll, between two deflection rolls or between the rolls in a stretching and tension-leveling unit, the oscillation of the strip is measured perpendicularly to the plane of the strip and used to determine one or more natural frequencies of this oscillation of the metal strip. A device according to the invention has a distance sensor for measuring the oscillation of the metal strip and a frequency analyzer for determining one or more natural frequencies of the oscillation of the metal strip perpendicularly to the plane of the strip on the basis of the excursion of the metal strip. In this way, a particularly precise determination of the strip tension or the tensile stress in the metal strip is possible, in particular even in the case of small strip tensions.

[0008] In an advantageous refinement of the invention, the natural frequency of the metal strip between the two bearing points, for example between two roll stands, between a roll stand and a coiler, between a roll stand and a deflection roll, between two deflection rolls or between the rolls in a stretching and tension-leveling unit, is determined, in particular by means of a frequency analyzer, by determining a dominant frequency of an oscillation of the metal strip.

[0009] In a further advantageous refinement of the invention, the calculation of the tension or the tensile stress in the metal strip takes place in particular by means of a tension calculator on the basis of the natural frequency of the metal strip.

[0010] In a further advantageous refinement of the invention, the tensile stress σ in the metal strip is determined according to

σ=1² cρf ²,

[0011] where

[0012] l is the length of the metal strip between the two roll stands, between the roll stand and the coiler, in the breakdown stand or in front of or behind the deflection roll,

[0013] c is a constant,

[0014] ρ is the density of the metal strip and

[0015] f is the natural frequency of the metal strip.

[0016] In a further advantageous refinement of the invention, the tension s in the metal strip is determined according to

s=1² cρhbf ²,

[0017] where

[0018] l is the length of the metal strip between the two roll stands, between the roll stand and the coiler, in the breakdown stand or in front of or behind the deflection roll,

[0019] c is a constant,

[0020] ρ is the density of the metal strip,

[0021] h is the thickness of the metal strip,

[0022] b is the width of the metal strip and

[0023] f is the natural frequency of the metal strip.

[0024] In a further advantageous refinement of the invention, the excursion of the metal strip is measured, in particular by means of excursion sensors, at different points of the metal strip, viewed in the transverse direction of the metal strip.

[0025] In a further advantageous refinement of the invention, at least $3\frac{b}{l}$

[0026] excursion sensors are provided for measuring the excursion of the metal strip, the excursion sensors being arranged in such a way that they measure the excursion of the metal strip at different points of the metal strip, viewed in the transverse direction of the metal strip. In this case,

[0027] l is the length of the metal strip between the two roll stands, between the roll stand and the coiler, in the breakdown stand or in front of or behind the deflection roll, and

[0028] b is the width of the metal strip.

[0029] In a further advantageous refinement of the invention, the at least two excursion sensors are arranged in such a way that they measure the excursion of the metal strip at least every 50 cm, viewed in the transverse direction of the metal strip.

[0030] In an advantageous refinement of the invention, the sensors or the sensor line is/are arranged in the longitudinal direction of the strip, preferably at the points of the maximum amplitude of the form of oscillation assigned to the natural frequency concerned, i.e. at ½ in relation to the first natural frequency, at ¼ in relation to the second natural frequency, etc.

[0031] In a further advantageous refinement of the invention, the tension in the metal strip is determined from measured values determined by at least two excursion sensors, in particular by means of an averaging unit.

[0032] In a further advantageous refinement of the invention, a deflection roll is provided for deflecting the metal strip.

[0033] In a further advantageous refinement of the invention, an excursion device is provided for the periodic excursion of the metal strip, i.e. in particular for producing a flexural wave, in particular a flexural wave in the longitudinal direction of the metal strip.

[0034] In a further advantageous refinement of the invention, the excursion device makes the metal strip undergo an excursion with a frequency which lies in the range of the natural frequency.

[0035] In a further advantageous refinement of the invention, a controller is provided for controlling the excursion device on the basis of a value determined by means of the excursion sensor.

[0036] Further details and advantages emerge from the following description of exemplary embodiments, in which specifically:

[0037]FIG. 1 shows a device for determining the tension or the tensile stress in a metal strip,

[0038]FIG. 2 shows the arrangement of excursion sensors on a measuring bar,

[0039]FIG. 3 shows an alternative device for determining the tension or the tensile stress in a metal strip,

[0040]FIG. 4 shows a further alternative device for determining the tension or the tensile stress in a metal strip and

[0041]FIG. 5 shows a further alternative device for determining the tension or the tensile stress in a metal strip.

[0042]FIG. 1 shows a device for determining the tension s or the tensile stress σ in a metal strip 1, that runs out of a roll stand 4 and is wound up onto a coiler 6. Arranged between the roll stand 4 and the coiler 6 is a deflection roll 5. Between the roll stand 4 and the deflection roll 5, the metal strip 1 oscillates or vibrates. This is represented by the three lines denoted by 1, which symbolize the metal strip. Provided underneath the metal strip 1 is an excursion sensor 2. The excursion sensor 2 may also be arranged above the metal strip 1. By means of the excursion sensor 2, the excursion of the metal strip 1 between the roll stand and the deflection roll 5 is sensed and transmitted to a frequency analyzer 7. The frequency analyzer 7 determines, for example by an FFT, the frequency spectrum of the measuring signal supplied by the excursion sensor 2. From the frequency spectrum, the frequency analyzer 7 determines the dominant fundamental oscillations and outputs them as the natural frequency f of the metal strip 1 between the roll stand 4 and the deflection roll 5. The natural frequency f of the metal strip 1 between the roll stand 4 and the deflection roll 5 is the input variable in a tension calculator 8.

[0043] The tension calculator 8 determines the tensile stress σ in the metal strip 1 according to

σ=1² cpf ²,

[0044] where

[0045] l is the length of the metal strip 1 between the roll stand 4 and the deflection roll 5,

[0046] c is a constant,

[0047] ρ is the density of the metal strip 1 and

[0048] f is the natural frequency of the metal strip 1 between the roll stand 4 and the deflection roll 5.

[0049] It may be provided that, instead of or in addition to the tensile stress σ, the tension calculator 8 calculates the tension 5 in the metal strip 1. In this case, the tension calculator determines the tension s in the metal strip 1 according to

s=1² cρhbf ²,

[0050] where

[0051] h is the thickness of the metal strip 1 and

[0052] b is the width of the metal strip 1 (cf. FIG. 2).

[0053] In an advantageous refinement of the device for determining the tension s or the tensile stress σ in the metal strip 1, it is provided that the excursion sensor 2 is part of a measuring bar 3. A measuring bar 3 of this type advantageously has a number of excursion sensors 2, 21, 22, 23 and 24, as shown in FIG. 2. FIG. 2 shows in this respect a plan view of the metal strip 1. A measuring bar advantageously has at least n excursion sensors for measuring the excursion of the metal strip, where $n \geq {3\frac{b}{1}}$

[0054]FIG. 3 shows a particularly advantageous exemplary embodiment of a device for determining the tension s or the tensile stress a in a metal strip 1, the metal strip 1 being represented in cross section. In this case, the measuring signals of the excursion sensors 2, 21, 22, 23 and 24, which are part of a measuring bar 3, are evaluated in a frequency analyzer 31. The frequency analyzer 31 forms spectral analyzes of the measuring signal determined by the excursion sensors 2, 21, 22, 23 and 24, determines their dominant fundamental frequency and forms an average value from the individual dominant fundamental frequencies. This average value is output as the natural frequency f. A frequency analyzer 7 according to FIG. 1 is designed in the same way if it evaluates the signals of a number of sensors.

[0055] In an advantageous refinement, a width measuring device 33 is provided for measuring the width b at the time of the metal strip 1. Furthermore, in an advantageous refinement, a thickness measuring device 34, that measures the thickness h at the time of the metal strip 1 in various measuring zones 35, is provided. The natural frequency f, the thickness h of the metal strip 1 and the width b of the metal strip 1 are input variables in a tension calculator 32. The tension calculator calculates the tensions and/or the tensile stress a in a way analogous to the tension calculator 8 in FIG. 1.

[0056] In an alternative refinement, it is provided that, instead of the one natural frequency f, the frequency analyzer 31 outputs the dominant fundamental frequencies in the measured values of the individual excursion sensors 2, 21, 22, 23, 24. In this alternative refinement, the individual significant fundamental frequencies are converted by means of the tension calculator 32 into associated tensile stresses σ and these are used to form an average value of the tensile stress with the aid of an averaging unit. The average value of the tensile stress, and the tension s derived from it, are output.

[0057]FIG. 4 shows a further advantageous exemplary embodiment and a device for measuring the tension s or the tensile stress a in a metal strip 1. For this purpose, in comparison with the exemplary embodiment in FIG. 1, an additional deflection roll 40 is provided between the excursion sensor 2 and the roll stand 4. The excursion sensor 2 measures the excursion of the metal strip 1 between the deflection roll 40 and the deflection roll 5. This measure has the effect of reducing the length 1 of the freely oscillating part of the metal strip 1. This shorter freely oscillating part of the metal strip 1 has a higher natural frequency f. This has the effect that the natural frequency f can be determined more quickly.

[0058]FIG. 5 shows a further advantageous exemplary embodiment of a device for determining the tension s or the tensile stress σ in a metal strip 1. This exemplary embodiment has an excursion device 53, which imposes an excursion on the metal strip 1. A particularly advantageous exemplary embodiment of such an excursion device can be taken, for example, from DE 198 03 260 A1. FIG. 5 shows the use of the excursion device 53 in conjunction with the exemplary embodiment according to FIG. 4. The excursion device may, however, also be used in conjunction with the other exemplary embodiments. The excursion device 53 makes the metal strip 1 undergo a periodic excursion, the frequency of the periodic excursion lying in the vicinity of the natural frequency f of the metal strip 1 between the deflection rolls 5 and 40. In a particularly advantageous refinement, a controller 52 is provided, said controller controlling the excursion device 53 on the basis of the natural frequency f determined by the frequency analyzer in such a way that the periodic excursion takes place by the excursion sensor 53 with the determined natural frequency f. In a particularly advantageous refinement, the excursion device 53 and the excursion sensor 2 are at the same time part of a device for measuring the tensile stress distribution in the metal strip 1, as disclosed in DE 198 03 260 A1. In an advantageous refinement in this respect, a further excursion sensor 51, which is arranged in the longitudinal direction of the part-strip 1 and may be part of a measuring bar 50, is provided. 

1. A device for determining the tensile stress (σ) or the tension (s) in a metal strip (1) between two bearing points (40) and (5), for example between two roll stands, between a roll stand (4) and a coiler (6), between a roll stand (4) and a deflection roll (5) or between the rolls in a stretching and tension-leveling unit, with an excursion sensor (2, 21, 22, 23, 24) for measuring the oscillation of the metal strip (1) perpendicularly to the plane of the strip, characterized in that the device for determining the tensile stress (σ) or the strip tension (s) includes a frequency analyzer (7, 13) for determining the natural frequency of the metal strip, taking the dominant frequency into account.
 2. The device as claimed in claim 1, characterized in that the frequency analyzer (7, 31) is formed such that its determines the natural frequency of the metal strip (1) between the two roll stands, between the roll stand (4) and the coiler (6), in the breakdown stand or in front of or behind the deflection roll (5, 40), by determining a dominant frequency of an oscillation of the metal strip (1).
 3. The device as claimed in claim 1 or 2, characterized in that it has a tension calculator (8, 32) for calculating the tension (s) or the tensile stress (σ) in the metal strip (1) on the basis of the natural frequency of the metal strip (1).
 4. The device as claimed in claim 3, characterized in that the tension calculator (8, 32) determines the tensile stress a in the metal strip (1) according to σ=1² cρf ², where l is the length of the metal strip (1) between the two roll stands, between the roll stand (4) and the coiler (6), in the breakdown stand or in front of or behind the deflection roll (5, 40), c is a constant, ρ is the density of the metal strip (1) and f is the natural frequency of the metal strip (1).
 5. The device as claimed in claim 3 or 4, characterized in that the tension calculator (8, 32) determines the tension s in the metal strip (1) according to s=1² cρhbf ², where l is the length of the metal strip (1) between the two roll stands, between the roll stand (4) and the coiler (6), in the breakdown stand or in front of or behind the deflection roll (5, 40), c is a constant, ρ is the density of the metal strip (1), h is the thickness of the metal strip (1), b is the width of the metal strip (1) and f is the natural frequency of the metal strip (1).
 6. The device as claimed in claim 1, 2, 3, 4 or 5, characterized in that it has at least two excursion sensors (2, 21, 22, 23, 24) for measuring the excursion of the metal strip (1), the at least two excursion sensors (2, 21, 22, 23, 24) being arranged in such a way that they measure the excursion of the metal strip (1) at different points of the metal strip (1), viewed in the transverse direction of the metal strip (1).
 7. The device as claimed in claim 6, characterized in that it has at least $3\frac{b}{l}$

excursion sensors (2, 21, 22, 23, 24) for measuring the excursion of the metal strip (1), the excursion sensors (2, 21, 22, 23, 24) being arranged in such a way that they measure the excursion of the metal strip (1) at different points of the metal strip (1), viewed in the transverse direction of the metal strip (1), where l is the length of the metal strip (1) between the two roll stands, between the roll stand (4) and the coiler (6), in the breakdown stand or in front of or behind the deflection roll (5, 40), and b is the width of the metal strip (1).
 8. The device as claimed in claim 6 or 7, characterized in that it has at least two excursion sensors (2, 21, 22, 23, 24) for measuring the excursion of the metal strip (1), the at least two excursion sensors (2, 21, 22, 23, 24) being arranged in such a way that they measure the excursion of the metal strip (1) at least every 50 cm, viewed in the transverse direction of the metal strip (1).
 9. The device as claimed in claim 6, 7 or 8, characterized in that it is formed such that it determines the tensile stress (σ) from measured values determined by the at least two excursion sensors (2, 21, 22, 23, 24), by means of an averaging unit.
 10. The device as claimed in one of the preceding claims, characterized in that it has a deflection roll (5, 40) for deflecting the metal strip (1).
 11. The device as claimed in one of the preceding claims, characterized in that it has an excursion device for the periodic excursion of the metal strip (1).
 12. The device as claimed in claim 11, characterized in that the excursion device makes the metal strip undergo an excursion with a frequency which lies in the range of the natural frequency.
 13. The device as claimed in claim 11 or 12, characterized in that it has a controller for controlling the excursion device on the basis of a value determined by means of the excursion sensor (2, 21, 22, 23, 24).
 14. The device for determining the strip tension as claimed in one of the preceding claims, characterized in that the excursion sensors (2, 21, 22, 23, 24) are arranged in the longitudinal direction of the metal strip (1) at the points at which the maximum amplitude of the characteristic form of oscillation of the natural frequency used for the stress determination occurs.
 15. A method for determining the tension (s) or the tensile stress (σ) in a metal strip (1) between two roll stands, between a roll stand (4) and a coiler (6), in a breakdown stand or in front of or behind a deflection roll (5, 40), in particular by means of a device as claimed in one of the preceding claims, the excursion of the metal strip (1) being measured, characterized in that the natural frequency of the metal strip (1) is determined on the basis of the excursion of the metal strip (1).
 16. The method as claimed in claim 15, characterized in that the natural frequency of the metal strip (1) is determined with the aid of of a maximum amplitude which occurs in the analysis of the respective natural frequency range. 